Let me be clear from the start – I don't think Mazzarri should be fired. Nor do I think he is a terrible coach. He's done relatively well with a really mediocre squad, and I think we've got a structure that wasn't present last season.
Mazzarri certainly hasn't been perfect. But for all the complaints about Mazzarri's refusal to play two strikers or his lack of faith in Belfodil and Kovačić, it's his defensive system that's actually been the biggest problem.
And I don't think all the goals can be blamed on 'individual errors.' All the goals we conceded against Napoli have a common pattern.
Napoli's first goal
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It’s easy to fault Yuto Nagatomo for the first goal – heading into the box is a terrible individual error. But it’s more important to look at what’s happening around him. Six Inter players are in the box, four Napoli players lurk just outside. Not a single Napoli player is marked. The theme is established pretty quickly – all the focus is on the ball, and midfield runners are left unattended to.
Higuain was the goalscorer, but it could've been any Napoli player around the box.
Napoli's second goal
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The second goal is a similar pattern. Again, you might fault individual errors – Ranocchia was burnt for pace. But the bigger issue is Dries Mertens’s run. Hugo Campagnaro halfheartedly runs back, but he might as well not be there for all the space that Mertens is given to score. Note also Jonathan’s approach toward the ball, leaving his marker (Insigne?) free.
Napoli's third goal
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Rinse and repeat for Napoli’s third. All the defenders are focused on Mertens winding up for his long shot. Saphir Taïder at least does the job of tracking Blerim Džemaili but doesn’t actually do anything defensively. By the team Taïder and Ranocchia have actually turned attention to Džemaili, the Swiss midfielder has already scored and wheeled away in celebration.
Napoli's fourth goal
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And it’s not much different for the fourth. Although the still image makes it look like Callejon is being marked by Rolando and Nagatomo, in truth the Spaniard has made a completely unmarked run from outside the box. He slips in between the two Inter defenders without being troubled by either one of them to convert Insigne’s low cross.
In every single Napoli goal, it’s the exact same pattern. Inter players are drawn to the ball, a Napoli player makes a vertical run from midfield, the Napoli player scores. For the second and third goals, Campagnaro and Taïder at least track their players, but in either case no pressure is put upon them. And the pattern isn’t unique to this game – take, for example, Totti’s opener in Roma’s 0-3 win.
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Again, all the focus is on Gervinho on the ball, while Totti and two other Roma players are left completely unmarked despite the presence of seven outfield Inter players.
So where does this put Mazzarri? Why has Mazzarri been unable to transfer his Napoli side’s defensive solidity? Has Mazzarri changed his formula?
The problem is that Mazzarri hasn’t changed, but the squad he’s managing has. Napoli's system under Mazzarri focused on energetic box-to-box midfielders running back to help protect the defense. We see elements of that here at Inter – look at Taïder attempting to track Džemaili. And individual errors have certainly played a part – Nagatomo should've not headed the ball into the box, Campagnaro should've done better against Mertens.
But when you see the same type of goal conceded again and again, it suggests there's a bigger problem. To put it simply, Mazzarri doesn't have the box-to-box midfielders – Inler, Džemaili, Behrami – he had, yet he sets out his team as though he does. This is a problem, because it leaves Inter far too open in the middle. Only Taïder is the type of midfielder that allows Mazzarri's defensive system to function. Cambiasso is too immobile, Guarín is undisciplined (and plays close to the striker anyways), and Álvarez and Kovačić work hard but aren't defensively great. Mazzarri is trying to force his midfielders to do a job that they simply are not good at, and Inter are too defensively slack as a result.
That's also an indictment of Inter's transfer policy, since they were unable to recruit properly for Mazzarri. But Mazzarri also has the responsibility adapt his tactics to fit the squad he does. Mazzarri's clearly a tactically adept and clever manager, so his failure to adapt thus far just further highlights his stubborn outlook.